In order to continue improving the spectral efficiency of GSM/EDGE Global (System for Mobile Communication (GSM) Enhanced Data for GSM Evolution), there is a need to find effective ways of multiplexing multiple users' signals into a single time slot over a 200 kHz channel while avoiding or minimizing mutual interference among users.
In accordance with a proposal in 3GPP/GERAN (third generation partnership project/GSM EDGE Radio Access Network a fast feedback channel for Voice over IP is proposed, see GERAN#44 GP-091988 “Fast Feedback Channel” (v1). The idea is to allow the Voice over internet protocol (IP) users to signal the network when there are packets available for transmission. This channel must allow fast feedback and must consume as little bandwidth as possible. To this end, several users are multiplexed into one timeslot by means of time division multiplexing.
Another example is Voice services over Adaptive Multi-user channels on One Slot (VAMOS), in which up to two full rate users are multiplexed in the same time slot. VAMOS is a standardized 3GPP/GSM feature. However, in the uplink, the receiver requires complex multi-user detection since the two user signals are ordinary Gaussian minimum shift keying (GMSK) co-channel interferers.
Due to the scarcity of the electromagnetic spectrum, it is desirable to multiplex more than 2 users into a single time slot, while keeping the receiver complexity low. Moreover, it is desirable to allow the users to employ GMSK modulation. This non-linear modulation technique has very good spectral properties and it is well suited for energy efficient analog RF front ends. Energy efficiency is always important in mobile stations.
Multi-user multiplexing has been standardized in GSM. In the uplink the users transmit using GMSK modulation, and become ordinary co-channel interferers. In the downlink AQPSK modulation is used, see 3GPP TS 45.004 v9.1.0. Note that neither the uplink nor the downlink transmission modes in VAMOS create truly orthogonal sub-channels.
The basic principle of VAMOS can not be easily extended to more than two users. For example in the uplink, a variant of VAMOS for 4 users would create 4 co-channel interferers. A legacy Base Transceiver Station (BTS) receiver with one or two antennas would have very poor performance. Furthermore, since some form of joint or sequential detection is required, the receiver algorithms would be quite complex.
Hence there exists a need for an improved transmission method.